U.S. patent number 10,608,795 [Application Number 16/011,393] was granted by the patent office on 2020-03-31 for apparatus and method for reception and transmission of control channels.
This patent grant is currently assigned to Motorola Mobility LLC. The grantee listed for this patent is Motorola Mobility LLC. Invention is credited to Vijay Nangia, Murali Narasimha, Ajit Nimbalker, Ravikiran Nory.
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United States Patent |
10,608,795 |
Nory , et al. |
March 31, 2020 |
Apparatus and method for reception and transmission of control
channels
Abstract
A method and system of creating, transmitting, receiving and
interpreting a subframe structure used in the communication between
a base station and a device with mobile communication functionality
is provided for use during communication on an unlicensed frequency
spectrum. The system and subframe structure provides for the
truncation of OFDM symbols within one or more subframe structures
that often contain downlink control information and a method for
determining a new location of the truncated downlink control
information within the subframe structure.
Inventors: |
Nory; Ravikiran (Buffalo Grove,
IL), Nangia; Vijay (Woodridge, IL), Narasimha; Murali
(Vernon Hills, IL), Nimbalker; Ajit (Buffalo Grove, IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
Motorola Mobility LLC |
Chicago |
IL |
US |
|
|
Assignee: |
Motorola Mobility LLC (Chicago,
IL)
|
Family
ID: |
55754396 |
Appl.
No.: |
16/011,393 |
Filed: |
June 18, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180309548 A1 |
Oct 25, 2018 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14733980 |
Jun 9, 2015 |
10009153 |
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62110404 |
Jan 30, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L
5/0053 (20130101); H04L 27/2602 (20130101); H04L
5/008 (20130101); H04L 5/001 (20130101); H04L
5/1469 (20130101); H04W 72/042 (20130101); H04W
72/0446 (20130101); H04W 16/14 (20130101); H04L
5/0051 (20130101) |
Current International
Class: |
H04L
5/00 (20060101); H04L 5/14 (20060101); H04W
16/14 (20090101); H04L 27/26 (20060101); H04W
72/04 (20090101) |
Primary Examiner: Mesfin; Yemane
Assistant Examiner: Baron; Henry
Attorney, Agent or Firm: Loppnow & Chapa Loppnow;
Matthew C.
Claims
We claim:
1. A method comprising: receiving, by a user equipment, a subframe
in a subframe structure from a base station, the subframe
comprising a first time slot comprising a first number (m1) of
orthogonal frequency-division multiplexed symbols and a second time
slot comprising a second number (m2) of orthogonal
frequency-division multiplexed symbols, wherein the first time slot
and the second time slot are non-overlapping; monitoring for
downlink control information in a first set of control channel
elements within the first number (m1) of orthogonal
frequency-division multiplexed symbols of the first time slot,
where m1 is an integer and where 1.ltoreq.m1.ltoreq.3; monitoring
for downlink control information in a second set of control channel
elements within the first number (m2) of orthogonal
frequency-division multiplexed symbols of the second time slot,
where m2 is an integer and where 1.ltoreq.m2.ltoreq.3; and
determining resulting downlink control information from one of the
first and second set of control channel elements.
2. The method according to claim 1, wherein the first number of
orthogonal frequency-division multiplexed symbols consists of seven
orthogonal frequency-division multiplexed symbols and the second
number of orthogonal frequency-division multiplexed symbols
consists of 7 orthogonal frequency-division multiplexed
symbols.
3. The method according to claim 1, wherein the base station is an
enhanced Node-B.
4. The method according to claim 1, wherein the downlink control
information in the second set of control channel elements is
determined to be the resulting downlink control information when
the subframe is a subframe with truncation that includes truncation
of at least one of the first number of orthogonal
frequency-division multiplexed symbols in the first time slot.
5. The method according to claim 1, wherein receiving further
comprises receiving the subframe from a serving cell in an
operating channel of an unlicensed frequency band.
6. The method according to claim 1, wherein the resulting downlink
control information from either the first set or the second set of
control channel elements comprises resource allocation information
prescribing an allocation of resources for physical downlink shared
channel reception.
7. The method according to claim 1, wherein when the resulting
downlink control information is determined to be from the second
monitoring, then the second set of control channel elements
comprises resource allocation information allocating resources in
the second number of orthogonal frequency-division multiplexed
symbols in the second time slot.
8. The method according to claim 1, wherein monitoring for downlink
control information in the second set of control channel elements
is performed if the monitoring for downlink control information in
the first set of control channel elements fails to find downlink
control information.
9. The method according to claim 1, wherein the determined
resulting downlink control information is from the first monitoring
of orthogonal frequency-division multiplexed symbols in the first
time slot and comprises resource allocation information prescribing
an allocation of resources in another plurality of orthogonal
frequency-division multiplexed symbols in a second subframe,
wherein the second subframe occurred immediately prior to the
subframe.
10. The method according to claim 1, wherein the determined
resulting downlink control information is from monitoring downlink
control information in the first set of the control channel
elements within the first orthogonal frequency-division multiplexed
symbols of the first time slot and wherein the resulting downlink
control indication comprises an indication value that indicates
that the subframe is a subframe with truncation and indicates a
number of symbols in the truncated portion of the subframe.
11. An apparatus comprising: a transceiver that receives a subframe
in a subframe structure from a base station, wherein the subframe
comprises a first time slot comprising a first number (m1) of
orthogonal frequency-division multiplexed symbols and a second time
slot comprising a second number (m2) of orthogonal
frequency-division multiplexed symbols, wherein the first time slot
and the second time slot are non-overlapping; and a processor that
monitors for downlink control information in a first set of control
channel elements within the first number (m1) of orthogonal
frequency-division multiplexed symbols of the first time slot,
where m1 is an integer and where 1.ltoreq.m1.ltoreq.3, monitors for
downlink control information in a second set of control channel
elements within the first number m2 of orthogonal
frequency-division multiplexed symbols of the second time slot,
where m2 is an integer and where 1.ltoreq.m2.ltoreq.3, and
determines downlink control information from one of the first or
second set of control channel elements.
12. The apparatus according to claim 11, wherein the first number
of orthogonal frequency-division multiplexed symbols consists of
seven orthogonal frequency-division multiplexed symbols and the
second number of orthogonal frequency-division multiplexed symbols
consists of 7 orthogonal frequency-division multiplexed
symbols.
13. The apparatus according to claim 11, wherein the base station
is an enhanced Node-B.
14. The apparatus according to claim 11, wherein the downlink
control information in the second set of control channel elements
is determined to be the resulting downlink control information when
the subframe is a subframe with truncation that includes truncation
of at least one of the first number of orthogonal
frequency-division multiplexed symbols in the first time slot.
15. The apparatus according to claim 11, wherein the transceiver
receives the subframe from a serving cell in an operating channel
of an unlicensed frequency band.
16. The apparatus according to claim 11, wherein the resulting
downlink control information from either the first set or the
second set of control channel elements comprises resource
allocation information prescribing an allocation of resources for
physical downlink shared channel reception.
17. The apparatus according to claim 11, wherein when the resulting
downlink control information is determined to be from the second
monitoring, then the second set of control channel elements
comprises resource allocation information allocating resources in
the second number of orthogonal frequency-division multiplexed
symbols in the second time slot.
18. The apparatus according to claim 11, wherein the processor
monitors for downlink control information in the second set of
control channel elements if the monitoring for downlink control
information in the first set of control channel elements fails to
find downlink control information.
19. The apparatus according to claim 11, wherein the determined
resulting downlink control information is from the first monitoring
of orthogonal frequency-division multiplexed symbols in the first
time slot and comprises resource allocation information prescribing
an allocation of resources in another plurality of orthogonal
frequency-division multiplexed symbols in a second subframe,
wherein the second subframe occurred immediately prior to the
subframe.
20. The apparatus according to claim 11, wherein the determined
resulting downlink control information is from monitoring downlink
control information in the first set of the control channel
elements within the first orthogonal frequency-division multiplexed
symbols of the first time slot and wherein the resulting downlink
control indication comprises an indication value that indicates
that the subframe is a subframe with truncation and indicates a
number of symbols in the truncated portion of the subframe.
Description
TECHNICAL FIELD
The following disclosure relates to wireless telecommunication
transmissions of Long Term Evolution (LTE) type communication
signals. In particular, the following disclosure relates to
approaches for truncating transmissions in the LTE subframe signal
and signaling options for the transmission of control channels and
reference signals.
BACKGROUND
LTE Advanced is being considered for deployment in the unlicensed
spectrum in the 5 GHz band. To do so, licensed spectrum in the
range 400 MHz to 3.8 GHz and spectrum in the unlicensed spectrum
bands would be aggregated together similar to the way that the
licensed spectrum bands are treated in standard LTE Advanced
carrier aggregation (CA). Regulatory authorities around the world
have defined or are defining a wide variety of requirements so that
various systems can inter-operate with incumbent users in the
unlicensed bands, notably including WiFi deployments.
For LTE Advanced (LTE Rel10-12) carrier aggregation (CA) or Dual
Connectivity, an eNB (E-Utran Node B) may be used. An eNB is
connected to a mobile phone network that communicates with mobile
handsets (UEs) and that configures a secondary serving cell (Scell)
to the UE to provide additional frequency resources (i.e., a
secondary carrier or a secondary component carrier (CC)) for
communication in addition to the primary serving cell (Pcell). The
Scell is typically activated for a UE at the onset of a data burst
transmission for the UE and deactivated after the transmission is
complete. Activation is done by using an activation command MAC
layer control element (MAC CE). Deactivation is done either upon
expiry of a deactivation timer or by using an explicit deactivation
command MAC CE. Additionally, a UE operating in LTE Advanced (LTE
Rel12) network can expect at least discovery signals from the eNB
on the Scell carrier while the carrier is deactivated. After
receiving an activation command for the Scell, the UE expects cell
specific reference signal (CRS) transmission from the eNB on the
Scell carrier from the subframe where activation command is
received till the subframe where the Scell is deactivated.
For further Advanced LTE (LTE Rel13) Scell, operation on unlicensed
carriers continues to be studied. The initial focus of the studies
appears to be Scell operation via a CA mechanism. However, it has
been perceived that some of the procedures identified for CA can
also be reused for dual connectivity (i.e., when the Scell and
Pcell belong to different cell groups).
Due to regulatory requirements, and due the need to for Advanced
LTE to co-exist with other wireless systems (e.g., Wi-Fi), LTE
devices (i.e., UEs and eNBs) need to take the following issues into
account while operating on an unlicensed carrier frequency.
First, before transmitting on an unlicensed carrier, the LTE
devices (e.g., eNB) typically have to check whether the carrier
(i.e., the carrier frequency) is busy using some form of `listen
before talk` (LBT) mechanism, then an LTE device can begin
transmissions only if the carrier is free. LBT typically comprises
measuring the energy on the carrier (sometimes referred to as
sensing) for a short duration (e.g., 9 us or 20 us) and determining
whether the measured energy is less than a threshold (e.g., -82 dBm
or -62 dBm). If the energy is less than the threshold, the carrier
is determined to be free. Some examples of LBT include the CCA-ED
(clear channel assessment-energy detect) and CCA-CS (clear channel
assessment-carrier sense) mechanisms defined in IEEE 802.11
specifications, CCA mechanisms specified in ETSI EN 301 893
specification etc.
And, second, transmissions on the carrier typically also have to
follow discontinuous transmission requirements (DCT requirements),
i.e., the LTE device can continuously transmit for Xms (e.g., X=4
ms as per some regulations, up to 13 ms for some other
regulations), after which it has to cease transmission for some
duration (sometimes referred as idle period), perform LBT, and
reinitiate transmission only if LBT is successful. The device may
perform LBT towards the end of the idle period.
As such, what is needed is a modification to certain transmissions
of LTE signals and on various channels that enable efficient
wireless network operation in both the licensed and unlicensed
carrier spectrums.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding, reference is now made to the
following description taken in conjunction with the accompanying
Drawings in which:
FIG. 1 illustrates a communications system in accordance with
various invention embodiments;
FIG. 2 illustrates a possible configuration of a computing system
to act as a base station;
FIG. 3 is a block diagram of an embodiment of a telecommunication
apparatus or electronic device that operates as a User Equipment
(UE);
FIG. 4A illustrates an example of a subframe structure having a
subframe with a truncated portion used for communication between a
base station and a UE;
FIG. 4B illustrates an example of the (n) subframe of FIG. 4A
showing S1 through s14 OFDM symbols during the subframe time
duration;
FIG. 5 illustrates another subframe structure having a truncated
portion during a terminal portion of one subframe and during a
first slot portion of the following subframe;
FIG. 6 illustrates another subframe structure having a truncated
portion during a terminal portion of one subframe and during a
portion of a first slot of the following subframe; and
FIG. 7 illustrates and example of Scell operation on an unlicensed
carrier frequency.
DETAILED DESCRIPTION
Embodiments will now be described more fully hereinafter with
reference to the accompanying drawings, in which various
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey pertinent aspects of
the invention to those skilled in the art.
As will be explained and discussed in more detail below, various
embodiments provide enhancements or modifications to the creation
and reception of LTE signals and channels in order to enable
operation in both licensed and unlicensed frequency spectrums.
In one embodiment a method is provided for a UE to receive a
subframe structure as a communication from a base station, such as
an eNB. The method comprises the UE receiving a subframe in the
subframe structure. The received subframe comprises a first time
slot comprising a first plurality of OFDM symbols and a second time
slot comprising a second plurality of OFDM symbols, wherein the
first time slot and the second time slot are non-overlapping time
slots. The UE monitors for downlink control information in a first
set of control channel elements (CCEs) with the first n2
(1.ltoreq.n2.ltoreq.3) OFDM symbols of the first time slot. The UE
also monitors for downlink control information in a second set of
control channel elements with the first n2 (1.ltoreq.n2.ltoreq.3)
OFDM symbols of the second time slot. The monitoring for downlink
control information may be done in parallel (i.e., at the same
time) or in series with the monitoring for downlink control
information. The UE then determines resulting downlink control
information from the monitoring of the first and second sets of
CCEs.
In some embodiments the first plurality of OFDM symbols consists of
seven OFDM symbols and the second plurality of OFDM symbols also
consists of seven ODFM symbols. In some embodiments, the downlink
control information in the second set of CCEs is determined to be
the resulting downlink control information when the subframe is a
subframe with truncation, which includes at least one of the first
plurality of OFDM symbols in the first time slot.
Additionally in various embodiments, receiving of the subframe
structure further comprises receiving the subframe from a serving
cell in an operating channel of an unlicensed frequency band.
Various embodiments may operate such that the resulting downlink
control information from either the first or the second set of CCEs
comprise resource allocation information prescribing an allocation
of resources for Physical Downlink Shared Channel (PDSCH)
reception.
In embodiments wherein the resulting downlink control information
is determined to be from the monitoring for downlink control
information in the second set of CCEs, then the second set of CCEs
comprises resource allocation information allocating resources in
the second plurality of OFDM symbols in the second time slot. In
some embodiments the monitoring for downlink control information in
the second set of CCEs is only done when the monitoring for control
information in the first set of CCEs in the first slot fails to
find any downlink control information therein. Alternatively, in
some embodiments wherein the determined resulting downlink control
information is from the monitoring for control information in the
first set of CCEs within the OFDM symbols in the first time slot,
the resource allocation information can comprise resource
allocation information prescribing an allocation of resources in
another plurality of OFDM symbols that are in a second subframe
that occurred immediately prior to the subframe from which the
resource allocation information was in.
In various embodiments the determined resulting downlink control
information is from monitoring downlink control information in the
first set of the CCEs within the first OFDM symbols of the first
time slot and wherein the resulting downlink control indication
comprises an indication value that indicates that the subframe is a
subframe control indication with truncation and indicates a number
of symbols in the truncated portion of the subframe.
Another embodiment provides a method for a base station to create a
subframe structure used in communication between the base station
and a UE wherein the subframe structure comprises a plurality
subframes that each have a subframe duration. The method for the
base station to create the subframe structure comprises the base
station operating a serving cell on a carrier frequency. The base
station performs a listen before talk (LBT) on the carrier
frequency for a LBT time duration. The LBT time duration at least
partially overlaps a terminal portion of a subframe duration of
first subframe within the plurality of subframes in the subframe
structure. The base station then determines where a position of a
starting OFDM symbol for transmitting a control channel in a second
subframe of the plurality of subframes should be placed based on
the LBT time duration of the LBT and wherein the subframe duration
of the second subframe occurs immediately after the subframe
duration of the first subframe.
In various embodiments, the base station transmits the control
channel starting from a determined position of the starting OFDM
symbol in the second subframe when a result of the LBT time
duration indicates that the carrier frequency is unoccupied during
the terminal part of the terminal portion of the first subframe
duration. Alternatively, the base station may transmit the control
channel starting at the eighth OFDM symbol (or at the beginning of
a second slot) of the second subframe when the result of the LBT
time duration indicates that the carrier frequency is occupied
during the terminal portion of the first subframe duration.
In yet another embodiment, a method for base station is provided
wherein the base station creates a subframe structure used in
communication between the base station and a user equipment (UE).
The subframe structure comprises a plurality of subframes that each
have a subframe duration. This embodiment, the base station
operator serving cell on a carrier frequency. The base station
performs a listen before talk (LBT) on the carrier frequency for a
time duration during at least a first portion of a first subframe
wherein the first portion of the first subframe occurs during a
terminal portion of the first subframe and possibly during at least
a second portion of a second subframe, wherein the second portion
of the second sub frame occurs at the beginning of the second
subframe and wherein the second subframe occurs immediately after
the first subframe. The base station then determines a position of
a starting OFDM symbol for transmitting a control channel in the
second subframe of the plurality of subframes based on the time
duration of the LBT. Also this embodiment the second subframe has a
second subframe duration that occurs immediately after the subframe
duration of the first subframe.
In various variations of this embodiment the position of the
starting OFDM symbol may be determined by determining that the
position of the starting OFDM symbol is at an initial OFDM symbol
position in the second subframe when the result of the LBT
indicates that the carrier frequency became unoccupied during a
terminal part of the first portion of the first subframe.
Determining the position of the starting OFDM symbol may also
include determining that the position of the starting OFDM symbol
is that and eighth OFDM symbol position in the second subframe when
the result of the LBT that the carrier frequency became unoccupied
during the second portion of the second subframe. Here the second
portion of the second subframe may comprise the initial through the
seventh OFDM symbol positions of the second subframe and wherein
the second subframe comprises fourteen OFDM symbol positions. In
some embodiments the base station also transmits the control
channel starting from the determined position of a starting OFDM
symbol.
Additionally in various embodiments the base station performs
physical downlink shared channel (PDSCH) transmissions to the UE in
the first subframe using the first UE-specific reference signal
pattern when the duration of the first portion is not greater than
the duration of the last three OFDM symbols of the first subframe.
The base station may also perform PDSCH transmission to the UE in
the first subframe using a second UE-specific reference signal
pattern when the duration of the first portion is greater than the
duration of the last three OFDM symbols of the first subframe.
In some embodiments the first UE-specific reference signal pattern
is a UE-specific reference signal pattern used for the LTE frame
structure type 2 (TDD) special subframe configurations 3, 4, 8, or
9; And wherein the second UE-specific reference signal pattern is a
UE-specific reference signal pattern used for LTE frame structure
type 2 (TDD) special subframe configurations 1, 2, 6 or 7.
Yet another embodiment of the invention provides a method for base
station to make a transmission between the base station and a UE on
an unlicensed carrier frequency. Here an exemplary method comprises
the base station operating a first serving cell on a licensed
carrier frequency and a second serving cell on an unlicensed
carrier frequency. The base station performs a listen before talk
(LBT) on the unlicensed carrier frequency for a first time
duration. The base station then determines whether the unlicensed
carrier frequency is free based on the LBT results. The base
station may then transmit an activation command on the first
serving cell for receipt by the UE. The activation command is
configured to activate the UE so that it receives transmissions
from the base station on the second serving cell. The base station
may also transmit, after determining that the unlicensed carrier
frequency is free, a reserve transmission on the unlicensed carrier
frequency at least until the base station begins transmission of
the activation command on the first serving cell.
In some embodiments the base station prepares the activation
command while transmitting the reserve transmission on the
unlicensed carrier frequency. Additionally in some embodiments, the
base station also schedules data to be transmitted to the UE on the
unlicensed carrier frequency for determined number of subframes.
The base station will discontinue transmission of scheduled data to
the UE after determined number of subframes and then perform
another LBT on the unlicensed carrier frequency for a second time
duration.
In yet another embodiment, a UE is configured to receive a subframe
structure that is used for communication from a base station to the
UE. In this embodiment the UE comprises a transceiver that's
configured to receive a subframe in a subframe structure, where in
the subframe structure comprises a first time slot comprising a
first plurality of orthogonal frequency division multiplexed (OFDM)
symbols and a second time slot comprising a second plurality of
OFDM symbols, wherein the first time slot and the second time slot
are non-overlapping. The UE further includes a processor that is
configured to monitor for downlink control information in a first
set of control channel elements (CCEs) within the first (m1)
(1.ltoreq.m1.ltoreq.3) OFDM symbols of the first time slot, where
(m1) is an integer. The processor is further configured to monitor
for downlink control information in a second set of control channel
elements (CCEs) within the first (m2) (1.ltoreq.m2.ltoreq.3) OFDM
symbols in the second time slot, where (m2) is an integer.
Additionally the processor is also configured to determined
downlink control information from one of the first or second sets
of CCEs.
FIG. 1 illustrates a communication system 100 including a network
102, a base station 104, such as an eNB, and a user equipment (UE)
106. The base station may also be referred to as a base unit, an
access point (AP), access terminal (AT), Node-B (NB), enhanced
Node-B (eNB), relay node, home eNB, pico eNB, femto eNB or by other
present or future terminology used in the art for a base station
derivation. Various communication devices may exchange data or
information through the network 102. The network 102 may be an
evolved universal terrestrial radio access (E-UTRA) or other type
of telecommunication network. A network entity, such as the base
station 104, may assign a UE identifier (UEID) to the UE 106 when
the UE 106 first joins the network 102. For one embodiment, the
base station 104 may be a distributed set of servers in the network
102. The UE 106 may be one of several types of handheld or mobile
communication devices, such as, a mobile phone, a laptop, or a
personal digital assistant (PDA). In one embodiment, the UE 106 may
be a wireless local area network capable device, a wireless wide
area network capable device, or any other wireless device.
FIG. 2 illustrates a possible configuration of a computing system
to act as the base station 104. The base station 104 may include a
processor/controller 210, a memory 220, a database interface 230, a
transceiver 240, input/output (I/O) device interface 250, and a
network interface 260, connected through bus 270. The base station
104 may implement any operating system, such as Microsoft
Windows.RTM., UNIX, or LINUX, for example. Client and server
software may be written in any programming language, such as C,
C++, Java or Visual Basic, for example. The server software may run
on an application framework, such as, for example, a Java.RTM.
server or .NET.RTM. framework.
The controller/processor 210 may be any programmable processor.
Various embodiments of the disclosure may also be implemented or
partially implemented on a general-purpose or a special purpose
computer, a programmed microprocessor or microcontroller,
peripheral integrated circuit elements, an application-specific
integrated circuit or other integrated circuits,
hardware/electronic logic circuits, such as a discrete element
circuit, a programmable logic device, such as a programmable logic
array, field programmable gate-array, or the like. In general, any
device or devices capable of implementing the decision support
method as described herein may be used to implement the decision
support system functions of this disclosure.
The memory 220 may include volatile and nonvolatile data storage,
including one or more electrical, magnetic or optical memories such
as a random access memory (RAM), cache, hard drive, or other memory
device. The memory may have a cache to speed access to specific
data. The memory 220 may also be connected to a compact disc-read
only memory (CD-ROM), digital video disc-read only memory
(DVD-ROM), DVD read write input, tape drive, solid state drive or
other removable memory device that allows media content to be
directly uploaded into the system. Data may be stored in the memory
220 or in a separate database (not specifically shown). The
database interface 230 may be used by the controller/processor 210
to access the database. The database may contain any formatting
data to connect the UE 106 to the network 102. The transceiver 240
may create a data connection with the UE 106. The transceiver 240
may configure a Physical Downlink Control Channel (PDCCH) and a
Physical Uplink Control Channel (PUCCH) between the base station
104 and the UE 106.
The I/O device interface 250 may be connected to one or more input
devices that may include a keyboard, mouse, pen-operated touch
screen or monitor, voice-recognition device, or any other device
that accepts input. The I/O device interface 250 may also be
connected to one or more output devices, such as a monitor,
printer, disk drive, speakers, or any other device provided to
output data. The I/O device interface 250 may receive a data task
or connection criteria from a network administrator.
The network connection interface 260 may be connected to a
communication device, modem, network interface card, a transceiver,
or any other device capable of transmitting and receiving signals
from the network 106. The network connection interface 260 may be
used to connect a client device to a network. The network
connection interface 260 may be used to connect the teleconference
device to the network connecting the user to other users in the
teleconference. The components of the base station 104 may be
connected via an electrical bus 270, for example, or linked
wirelessly.
Client software and databases may be accessed by the
controller/processor 210 from memory 220, and may include, for
example, database applications, word processing applications, as
well as components that embody the decision support functionality
of the present disclosure. The base station 104 may implement any
operating system, such as Microsoft Windows.RTM., LINUX, or UNIX,
for example. Client and server software may be written in any
programming language, such as C, C++, Java or Visual Basic, for
example. Although not required, the disclosure is described, at
least in part, in the general context of computer-executable
instructions, such as program modules, being executed by the
electronic device, such as a general purpose computer. Generally,
program modules include routine programs, objects, components, data
structures, etc. that perform particular tasks or implement
particular abstract data types. Moreover, those skilled in the art
will appreciate that other embodiments of the disclosure may be
practiced in network computing environments with many types of
computer system configurations, including personal computers,
hand-held devices, multi-processor systems, microprocessor-based or
programmable consumer electronics, network PCs, minicomputers,
mainframe computers, and the like.
FIG. 3 illustrates in a block diagram of one embodiment of a
telecommunication apparatus or electronic device to act as the UE
106. The UE 106 may be capable of accessing the information or data
stored in the network 102. For some embodiments of the disclosure,
the UE 106 may also support one or more applications for performing
various communications with the network 102. The UE 106 may be a
handheld device, such as, a mobile phone, a laptop, a personal
digital assistant (PDA) smart phone or other multi-function
communication device. For some embodiments, the UE 106 may be WiFi
capable device, which may be used to access the network 102 for
data or by voice using VOIP and WiFi carrier frequency
spectrums.
The UE 106 may include a transceiver 302, which is capable of
sending and receiving data over the network 102. The UE 106 may
include a processor 304 that executes stored programs or
applications that may, among other things, monitor, control and
interact with the other components of the UE 106. The UE 106 may
also include a volatile memory 306 and a non-volatile memory 308
which are used by the processor 304. The UE 106 may include a user
interface 310 that may comprise user input elements such as a
keypad, display, touch screen, and the like. The user interface 310
may also include user output elements that may comprise a display
screen and/or vibration and illumination indicators. The UE 106 may
also include an audio interface 312 that may comprise elements such
as a microphone, earphone, and speaker. The UE 106 also may include
a component interface 314 to which additional elements may be
attached, for example, a universal serial bus (USB) interface. The
UE 106 may include a power supply 316. In addition, the UE may be
incorporated as a peripheral or integral part of a larger system
such as a transportation vehicle, building, entertainment center,
kiosk, or gaming device to name a few.
In Long Term Evolution (LTE) communication systems, physical layer
signals and channels (e.g., a control channel like a physical
downlink control channel (PDCCH), enhanced physical downlink
channel (EPDCCH); a data channel like a physical downlink shared
channel (PDSCH); reference and synchronization signals like a
primary synchronization signal (PSS), secondary synchronization
signal (SSS), cell-specific reference signal (CRS), channel state
information reference signal (CSI-RS), and discovery signals are
transmitted by a base station 104 using orthogonal
frequency-division multiplexed (OFDM) symbols. For normal cyclic
prefix (CP) operation, the OFDM symbols are of -71 us in duration.
Seven OFDM symbols may comprise a 0.5 ms slot and two slots may
comprise a 1 ms LTE subframe. Therefore, an example of an LTE
subframe comprises 14 OFDM symbols.
In order to enable efficient secondary serving cell (Scell)
operation on an unlicensed carrier frequency, it is beneficial for
the base station or eNB 104 to not use some of the symbols or
portions of the symbols in (i.e., to truncate some of the symbols
or portions of the symbols) the transmission of physical layer
signals or channels in some of the subframes such that they
comprise less than 14 symbols, but still utilize the truncated
portion of the subframe (i.e., the remaining portion of the
subframe where the symbols are not being used) to perform a listen
before talk (LBT) function or as an idle period.
For example and referring to FIG. 4A, a subframe structure 400 used
for communication between an eNB 104 and a UE 106 is shown. In this
example, each subframe 402 comprises 14 symbol positions (s1-s14)
shown in FIG. 4B. In some embodiments, after every continuous
transmission of 2 subframes (e.g., subframes (n) and (n+1)), the
eNB can truncate the transmission of the 3.sup.rd subframe (n+2)
406 to be 11 symbols long and use the remaining truncated portion
404 (i.e., 3 truncated symbols) to perform LBT. Alternately in
other embodiments, after every continuous transmission of, for
example, 5 subframes, the eNB can truncate the transmissions of,
for example, the 6.sup.th subframe to 9 symbols and use the
remaining 5 symbols to perform LBT. Alternatively, in other
embodiments, after every continuous transmission of, for example,
11 continuous subframes, the eNB can truncate the transmission of
the 12.sup.th subframe to be, for example, 5 symbols long and use
the remaining truncated portion of 9 symbols to perform LBT. These
example transmission approaches may be consistent with requirements
for load based equipment transmission in ETSI EN 301 893
specification.
In various embodiments, the eNB 104 may use some or the entire
truncated portion 404 of the subframe with truncation 406, for
example (n+2), to perform LBT. The eNB 104 may also use some part
of the truncated portion 404 of the subframe with truncation 406 as
an idle period (e.g., the duration of the idle period may be
similar to the DIFS period described in IEEE 802.11
specifications). After the eNB 104 performs LBT and determines that
the carrier is free, the eNB 104 may also use some part of the
truncated portion 404 of the subframe with truncation 406 to
transmit a `carrier holding signal` or `carrier busy signal` until
the next LTE pilot/data/control transmission can begin to ensure
that other devices sensing the carrier detect that the carrier is
occupied by the eNB and defer their transmissions (not specifically
shown). The `carrier holding signal` may be transmitted with the
same power level or PSD level as the subsequent LTE
pilot/data/control transmission. The `carrier holding signal` may
be a narrowband signal or a wideband signal and may/may not be
centered within the operating channel on the unlicensed carrier
frequency.
From a UE 106 perspective, it is beneficial for the UE 106 to be
able to determine or to be provided information defining which of
the received subframes in the subframe structure 400 are subframes
with truncated transmissions 406 and the number (m) of OFDM symbols
truncated in the truncated portion 404 of the subframes with
truncation 406. If this information is not made available to the
UE, the UE may attempt to receive the signals/channels transmitted
by the eNB in a manner that assumes that the subframes therein do
not have any truncation, which will lead to estimation/decoding
errors that will cause retransmissions and inefficient carrier
usage.
Subframes with truncation 406 are typically done only after
continuous transmission of a certain number of subframes 402. The
number of subframes continuously transmitted prior to a subframe
with truncation 406 depends on the transmission activity of the eNB
104, which in turn depends on the data arrival patterns for various
UEs served by the eNB. Given this, the subframes with truncation
406 may not follow a periodic pattern in some embodiments. Further,
the subframes that are subframes with truncation may also be
impacted by the variability of the duration of the idle period or
LBT observation period due to, e.g., random backoff when the
carrier is detected as being occupied. Also, the number of symbols
in the truncated portion 404 in a given subframe with truncation
406 can vary depending on operating parameters chosen by the eNB
104. After considering these aspects, the following approaches are
possible techniques or methods for providing signaling to a UE 106
to indicate which subframe(s) is a subframe with truncation and
what portion of the subframe(s) with truncation is the truncated
portion.
Still referring to FIG. 4A, a first example of an approach for
signaling a UE 106 to indicate that there are subframes with
truncation is for the UE to be configured by higher layers (e.g., a
radio research control (RRC) message) with a set of values wherein
each value corresponds to a number of truncated symbols at the end
of a subframe with truncation. When the UE 106 is assigned a PDSCH
in a subframe and when that subframe is a subframe with truncation
406, the number of symbols (m) in the truncated portion 404 of that
subframe with truncation is indicated in the associated downlink
control information (DCI, which may be part of the PDCCH), that
assigns the PDSCH. For example, the set of symbols s={0, 3, 5, 9}
can be configured by higher layers such that two bits in the DCI
can signal one of the four values {0, 3, 5, 9} as being the number
of symbols (m) in the truncated portion 404. In another example, a
set {0, 3} can be configured by the eNB such that one bit in the
DCI indicates whether the subframe is not truncated {0} or has a
truncated portion of 3 symbols (m=3). In yet another example, one
bit can be configured in the DCI to indicate whether the subframe
is or is not a subframe with truncation. Additionally the UE can be
configured by higher layers (e.g., a RRC message) with a truncated
symbol value corresponding to the number of truncated symbols that
are part of the truncated portion at the end of a subframe with
truncation. When symbols are truncated, the truncated portion is
comprised of contiguous or a set of serial symbols.
A second example of an approach or method for signaling a UE 106
for subframes with truncation 406 can be performed with a physical
downlink control channel (PDCCH) or an enhanced physical downlink
control channel (EPDCCH). Here the DCI contents of a PDCCH/EPDCCH
transmitted in the common search space of the primary cell (Pcell,
i.e., a cell operating on the licensed carrier) indicate the number
of symbols in the truncated portion of the corresponding subframe
with truncation of the Scell (operating on the unlicensed carrier).
In this approach, the PDCCH/EPDCCH can be decoded by all UEs and/or
a set of UEs operating on the unlicensed carrier. A special radio
network temporary identifier (RNTI) may be configured via higher
layers for scrambling the cyclic redundancy check (CRC) of the
associated DCI.
In a variant of the second example, the DCI contents of a
PDCCH/EPDCCH transmitted in the UE-specific search
space/carrier-specific search space of another cell (e.g., a
primary or secondary cell operating on the licensed carrier)
indicate the number of symbols that are truncated (i.e., a
truncation value) in the truncated portion at the end of the
subframe (i.e., the terminal part of the subframe) in the
corresponding subframe with truncation of the Scell (operating on
the unlicensed carrier). In this approach, the PDCCH can be decoded
by multiple UEs operating on the unlicensed carrier for which the
DCI is intended.
A third example of an approach or method for signaling a UE that
certain subframes are subframes with truncation and indicate which
symbols of the subframe with truncation are truncated as part of
the truncated portion can be accomplished as follows. Instead of
indicating the truncation value, the higher layers configure the UE
with one or more subframe types, each subframe type may correspond
to a subframe having a predetermined number of symbols in the
subframe, e.g., subframe type 1 corresponds to 14 OFDM symbols
(i.e., 1 ms duration) and using a first type of reference signal
structure, and subframe type 2 corresponds to 11 OFDM symbols
(11/14 ms duration) corresponding to a subframe with truncation
having a truncated portion of 3 ODFM symbols and using a second
type of reference signal structure, and so on.
In some scenarios, it is not possible for the eNB to truncate only
the last symbols (i.e., the terminal portion) of a subframe with
truncation. For example, if the eNB begins a LBT clear channel
assessment in the last symbols of a subframe with truncation (e.g.,
due to limitation on the maximum channel occupancy time), and if
the carrier is not detected as free until sometime in the next
subframe (i.e., the subframe immediately following a first subframe
with truncation), then the eNB can only start LTE transmission
somewhere in the middle of the next subframe.
FIG. 5 illustrates another subframe structure 500 used for
communication between a base station and a UE. Here the end or
terminal portion 504 of subframe n+2 and some of the beginning
portion of subframe (n+3) are truncated by the eNB into the
truncated portion 502. In such a scenario it is not possible for
the eNB to indicate which symbols are truncated symbols in the
beginning portion 506 of subframe (n+3) via control signaling 520
in subframe n+2 (because eNB will not know how long the LBT will
take) or the control signaling in subframe (n+3) (because the LBT
occurs before the subframe (n+3) signaling).
One option for the eNB 104 in this scenario is to truncate or not
transmit in the entire subframe (n+3) and then begin PDCCH/PDSCH
transmission in the following subframe after subframe (n+3) (i.e.,
subframe (n+4)). The eNB may use some part of the truncated portion
of subframe (n+3) to transmit a `carrier holding signal` after the
carrier is detected to be free in the truncated portion. However,
this option is inefficient as all resources in subframe (n+3) are
wasted (especially if truncation is required only for a few
beginning symbols of subframe (n+3)).
A more efficient option is to initiate control signaling
transmission in subframe (n+3) after the eNB determines that the
carrier is free. With this option, if the eNB uses PDCCH to assign
PDSCH resources to a UE in subframe (n+3), and if some or all of
the symbols of the first slot 510 of the subframe (n+3) have to be
truncated (e.g., due to an ongoing LBT), then the eNB can transmit
the PDCCH 516 starting from the first symbol of the second slot
512. In this example, each subframe 508 in the subframe structure
500 comprises 14 symbols wherein the first slot 510 of a subframe
508 contains the first 7 symbols (s1-s7) and the second slot 512 of
the subframe 508 contains the second 7 symbols (s8-s14). A slot
boundary 514 is shown as a dashed horizontal line in each subframe
508.
FIG. 5 illustrates subframe (n+3) where PDSCH 518 is transmitted in
the symbols in a second slot following PDCCH 516 because the first
slot 510 of the (n+3) subframe is part of the truncated portion 506
at the beginning of subframe (n+3).
Alternatively, in some circumstances PDSCH may span both symbols in
the second slot 512 of a first subframe with truncation (n+2) and
some symbols (e.g., the last two symbols or at least some symbols
following the instant in time when the carrier is detected to be
free towards the end of the first slot) in the first slot of a
second subframe with truncation (n+3) that immediately follows the
first subframe with truncation (n+2). For example, if the first 5
symbols in the first slot 510 of the second subframe with
truncation (n+3) are truncated, PDCCH 516 is transmitted in the
first symbol of the second slot 512 (i.e., the s8 symbol position
in the second subframe with truncation (n+3)) and PDSCH 518 is
assigned in the last two symbols of the first slot (6.sup.th and
7.sup.th symbol positions s6 and s7) of the second subframe with
truncation (n+3) and symbols in the second slot 512 of the second
subframe with truncation (n+3). This alternative has the advantage
of utilizing the subframe resources a little more efficiently
(assuming the first 5 symbols (m=5 and s=s1 through s5) of the
subframe are enough for truncation) and it is more suitable for
PDSCH assignments for transmission modes 8, 9, 10 that utilize
demodulation reference signals present in the s6 and s7 symbol
positions of the first slot of the subframe.
Additional efficiency may be provided wherein, the DCI of the PDCCH
516 in the subframe with truncation (n+3) can also indicate the
number of truncated symbols in that subframe at the beginning of
the subframe with truncation (n+3) so that PDSCH 518 is transmitted
according to DCI (e.g., if DCI indicates 3 symbols (s1-s3), PDSCH
518 can be transmitted in the remaining 11 symbols (s4-s14)).
From a UE perspective, a UE typically monitors for PDCCH candidates
in the first few symbols in the first slot 510 of a subframe 508
where monitoring implies attempting to blindly decode a PDCCH
candidate assuming a particular set of CCEs (control channel
elements) within the UE's PDCCH search space.
However, for the alternatives described above, the UE has to
monitor for PDCCH candidates in the first few symbols of both the
first slot 510 and the second slot 512 of each subframe 508. More
specifically, the UE performs b1 blind decodes (BDs) in one or more
symbols at the beginning of the first slot of a subframe with
truncation and b2 additional BDs in one or more symbols at the
beginning of the second slot of the subframe with truncation (b1
and b2 are integers). The UE may perform the additional b2 BDs only
if the first b1 BDs are not successful. For this operation, the UEs
PDCCH search space comprises two parts, the first part comprising
PDCCH candidates assuming sets of CCEs within the first m1 symbols
(1.ltoreq.m1.ltoreq.3) of the first slot of the subframe with
truncation and a second part comprising sets of CCEs within the
first m2 symbols (1.ltoreq.m2.ltoreq.3) of the second slot of the
subframe with truncation.
To indicate the span (i.e., the number of symbols) of PDCCH 516
transmission in the second slot, the eNB can also transmit a PCFICH
in the first symbol of the second slot of the second subframe with
beginning truncation (n+3). Alternatively, the span of PDCCH in the
second slot 516 can be fixed or configured (e.g., signaled by
higher-layers (e.g., a RRC message)) to 1 or 2 symbols (m=1 or 2)
to reduce UE complexity. In addition to PDCCH and PCFICH, PHICH may
also be transmitted at the beginning of the second slot 512 of the
subframe with beginning truncation (n+3). The RE (resource element)
locations for PCFICH can follow the same mapping as that of
existing LTE specifications (except for the change from first slot
to second slot). Alternatively, additional higher layer signaling
may help the UE determine the PCFICH and PDCCH RE locations. The
PHICH configuration is signaled in PBCH for the existing
specifications, whereas the PHICH for second slot may be separately
configured.
For the purpose of PDCCH decoding, and PDSCH RE mapping (if PDCCH
with PDSCH assignment for the UE is detected), the UE may assume
that CRS is present in the second slot 512 of the subframe with
truncation (n+3) even if that subframe with truncation is
configured as an MB SFN subframe.
Referring now to FIG. 6, if the eNB 104 uses EPDCCH to assign PDSCH
resources to a UE in subframe (n+3), and if some or all of the
symbols of the first slot 606 of the subframe (n+3) have to be
truncated (e.g., due to ongoing LBT), then the eNB can transmit the
EPDCCH starting from an alternative OFDM symbol location 607 that
begins after the truncation period 602.
In principle, the starting symbol for EPDCCH transmission in
subframe (n+3) can be dynamically varied by the eNB based on length
of the beginning portion 604 truncation period, at the beginning of
the subframe with truncation (n+3). However, this would result in
very high UE complexity (since UE has to blindly try all possible
starting locations).
A simpler alternative is for the eNB 104 to signal one additional
EPDCCH starting location to the UE 106 via RRC signaling if
necessary. In current LTE systems, the EPDCCH starting symbol
location is signaled to the UE via RRC (e.g., it can be 2.sup.nd,
3.sup.rd, 4.sup.th symbol in the first slot for a 20 MHz carrier).
In various embodiments, the eNB will additionally configure the UE
with an additional EPDCCH starting location (e.g., 5.sup.th,
6.sup.th or 7.sup.th symbol of the first slot 606, or the first
symbol of second slot 608) to accommodate alternative EPDCCH
transmission if the first few symbols of the first slot 606 of the
subframe with beginning truncation (n+3) are truncated.
An even simpler alternative (albeit less efficient than the above
two examples) comprises transmitting EPDCCH starting from the first
symbol of the second slot 608 if a higher layer configured starting
symbol (in the first slot 606) has to be truncated.
In various embodiments, PDSCH can be transmitted from the same
starting location as that of EPDCCH or DCI of the EPDCCH, and can
indicate the number of truncated symbols at the beginning of the
subframe with truncation (n+3) so that PDSCH is transmitted
according to DCI (e.g., if DCI indicates 3 truncated symbols, PDSCH
can be transmitted in the remaining 11 symbols).
From a UE perspective, the UE performs b1 BDs assuming a first
EPDCCH starting position configured by higher layers, and performs
b2 additional BDs assuming an alternative EPDCCH starting position
in a subframe. (Here b1 and b2 are integers.) The UE may perform
the additional b2 BDs only if the first b1 BDs are not successful.
The alternative EPDDCH starting position in a subframe may be fixed
(e.g., the s6 symbol position of first slot of each subframe), or
it may be configured via RRC.
In another embodiment, the UE performs b1 BDs assuming EPDCCH is
contained within the 1.sup.st slot, performs b2 additional BDs
assuming EPDCCH is contained within the 1.sup.st and 2.sup.nd slot,
and b3 BDs for EPDCCH contained within only 2.sup.nd slot. (Here
b1, b2 and b3 are each integers.) This is useful for cases where
the UE does not know if the subframe with truncation has a
truncated portion at the beginning or the end of the subframe,
i.e., the eNB has the flexibility to schedule the UE in 1.sup.st
slot only, 2.sup.nd slot only or 1.sup.st and 2.sup.nd slot
together.
If the beginning portion of a subframe has to be truncated (e.g.,
as shown in subframe (n+3) in FIG. 5), another alternative is for
the eNB 104 to indicate the truncated beginning portion 506 of a
given subframe with truncation (n+3) in the next subframe (n+4).
For example, when the first few beginning symbols of subframe (n+3)
shown in FIG. 5 are truncated, then instead of making the UE
determine the starting symbol of control channel transmission 516
in subframe (n+3) via the blind decoding approaches described
above, the eNB can signal the truncation information for subframe
(n+3) using DCI of a PDCCH/EPDCCH transmitted in subframe (n+4)
(not specifically shown). The DCI should have additional
information indicating that the truncation information applies to
the previous subframe (i.e., subframe n+3) and not the current
subframe (n+4). This can be provided by a subframe indicator bit(s)
in the DCI of subframe (n+4), or by using a configured
cross-subframe RNTI. The DCI can also provide resource allocation
information for the previous subframe (i.e., subframe n+3).
In current LTE systems, for receiving PDSCH transmissions for some
transmission modes (e.g., TM 8, 9, or 10), the UE uses UE-specific
demodulation reference signals (UERS) present in REs of the last
two symbols of a subframe for LTE frame structure type 1 (FDD).
However, for LTE operation on unlicensed carriers, in subframes
where the last two or more symbols are truncated, these UERS will
be unavailable to the UE, and utilizing only the remaining UERS in
other non-truncated symbols will result in degraded channel
estimation. Embodiments solve this problem by providing a more
efficient alternative. That is, for LTE frame structure type 1
(FDD), for transmitting UE-specific reference signals for antenna
ports 7, 8, 9 and 10:
(1) if the eNB has to provide a subframe with truncation wherein
only the last three or fewer symbols in an end portion 608 in a
subframe with truncation (n+2) are truncated using normal cyclic
prefix (CP), then for those subframes, the eNB uses the UE-specific
reference signals pattern described for LTE frame structure type 2
(TDD) special subframe configurations 3, 4, 8, or 9 in the 3GPP TS
36.211 vc.2.0 specification.
(2) if the eNB has to truncate more than last three symbols in the
second slot 610 of a subframe with truncation (n+2) using normal
CP, then for that subframe with truncation (n+2), the eNB uses the
UE-specific reference signals pattern described for LTE frame
structure type 2 (TDD) special subframe configurations 1, 2, 6, or
7 in the 3GPP TS 36.211 vc.2.0 specification.
This alternative is efficient as the UERS pattern for (TDD) special
subframe configurations 3, 4, 8, or 9 in the 3GPP TS 36.211 vc.2.0
specification does not include REs in the last three symbols of the
subframe. Similarly, the UERS pattern for (TDD) special subframe
configurations 1, 2, 6, or 7 does not include REs in the second
slot of the subframe.
The CSI-RS transmissions can also adapted to fit into the subframes
with truncation. The OFDM symbols in which CSI-RS are transmitted
are configured via higher layers. If an OFDM symbol in which CSI-RS
is to be transmitted corresponds to a truncated OFDM symbol, then
the UE may assume that the CSI-RS is not transmitted in the OFDM
symbol. In another approach, the CSI-RS configuration for truncated
subframes may be configured via separate higher layer
signaling.
In existing LTE systems Scells are typically operated on a carrier
(also sometimes referred as a `component carrier` or a `channel`)
using the following states:
(1) State where the carrier is not added as Scell ("not configured"
state):
Here the UE may be configured to perform inter-frequency
measurements on the carrier frequency layer using measurement gaps.
No additional power consumption is necessary due to Scell receiver.
The Scell receiver need not be turned on and measurements can be
performed using the Pcell receiver.
(2) State where the carrier is added as Scell but not activated
("deactivated state"):
Here the UE receives relevant system information for the Scell from
the Pcell. The UE may be configured to perform measurements on the
Scell without measurement gaps. There will be minimal additional
power consumption due to Scell receiver. The Scell receiver needs
to be occasionally turned on (e.g., for 5 ms every 160 ms) for RRM
measurements using either CRS or Discovery signals
(3) State where the carrier is added as Scell and activated
("activated state"):
Here the UE is expected to receive PDSCH on the Scell in all
activated subframes. There is high additional power consumption due
to the Scell receiver. Scell receiver should perform PDCCH
monitoring in every subframe (for self scheduling case). The Scell
receiver needs to buffer every subframe for potential PDSCH
processing (for both self and cross-carrier scheduling cases)
For Scell operation on unlicensed carriers, it is desirable to
retain the above states so that UE power consumption for unlicensed
carrier operation is similar to that of licensed carrier
operation.
However, it should be noted that some of the embodiments, when
compared to current LTE operation, are further required to enable
efficient operation taking into account the need for the eNB to
perform LBT and obey discontinuous transmission requirements (DCT
requirements).
FIG. 7 illustrates an example of Scell operation on an unlicensed
carrier frequency. In FIG. 7, the eNB operates (i.e., performs the
necessary steps to communicate with a UE) a Pcell 700 on a licensed
carrier. The eNB also operates a Scell 702 (denoted as uScell in
FIG. 7) on an unlicensed carrier. The UE receives transmissions
from the eNB on both the Pcell and uScell 704 (only uScell
reception is shown). Subframes indicated as "D" represent
deactivated state subframes for the UE. Subframes indicated as "A"
represent activated state subframes for the UE.
When a data burst to be transmitted on the uScell arrives in the
eNB buffer 701, the eNB starts LBT 706 on the unlicensed carrier.
After determining that the carrier is free 708, the eNB starts
transmitting 712 a reserving transmission on the unlicensed carrier
to effectively reserve the unlicensed carrier frequency. The
reserving transmission is made to ensure that other devices sensing
the carrier detect that the carrier is occupied by the eNB. It also
prepares an activation command 710 for transmission on the PCell to
the UE. When the eNB is ready to transmit the activation command
(e.g., after a one or two subframe delay for internal processing,
two subframe delay shown in the figure), it transmits the
activation command 710 on the Pcell. The UE, after receiving the
activation command 710, transitions to an unlicensed carrier
frequency receive activated state A from its deactivated unlicensed
carrier receive state D. The UE may need a few subframes (e.g., 1
or 2 subframes, 1 subframe shown in the figure) to make this
transition 714. Once the UE is in activated state, the eNB
schedules data (using PDSCH) to the UE. After transmitting for a
few subframes (e.g., 5 subframes shown in the figure), the eNB
stops transmission and reinitiates LBT 715 on the unlicensed
carrier to satisfy DCT requirements. After the carrier is
determined to be free 716, the eNB reinitiates PDSCH transmission
to the UE. This process is continued till the data burst
transmission is complete.
Preparation of activation command at the eNB includes determining
DCI payload bits, CCE locations and CCE aggregation level of the
PDCCH associated with the activation command. Some of the steps for
preparing the activation command can be performed in advance by the
eNB (e.g., determining payload bits). However, other steps (e.g.,
determining CCE locations for PDCCH transmission) can only be
performed after determining the subframe where the activation
command is transmitted 730, which in turn, depends on the subframe
in which the eNB determines the unlicensed carrier is free 732. For
example, if the eNB determines that the unlicensed carrier is free
in subframe n, it prepares for activation command transmission on
the Pcell in subframe (n+3). As explained earlier, between subframe
(n) and (n+3), the eNB continues transmitting on the unlicensed
carrier 712 to ensure that other devices sensing the carrier
realize that the carrier is occupied by the eNB. The transmissions
may include reference signal transmissions such as CRS or
CSI-RS.
To shorten the UE transition 714 from deactivated D to activated A
state, the activation command can be embedded in physical layer
signaling (e.g., within the DCI payload of a PDCCH), instead of the
higher layer MAC CE based approach (which involves both PDCCH and
PDSCH) used in current LTE systems.
The eNB has to transmit at least reference signals (e.g., CRS) on
the unlicensed carrier in the subframe 734 where the activation
command is transmitted and a few immediately following subframes
(e.g., one subframe 735 following the activation command). These
reference signals can be used by the UE (that is being activated)
for purposes such as regaining fine time/frequency sync and
automatic gain control (AGC) warm up.
From the UE perspective, it continues to be in activated state A
until it receives a deactivation command from the eNB or the
expiration of a deactivation timer configured by the eNB via higher
layers (whichever occurs earlier) 720, after which the UE
transitions 718 to the low power deactivated state.
As shown in the FIG. 7, for unlicensed carrier operation, the eNB
may not have any transmissions in some subframes 736 (e.g., due to
LBT procedure related delay) when the UE is in activated state.
This is different from current LTE systems where activated UEs
expect the eNB to transmit at least CRS in every subframe. The UEs
need to receive frequent reference signal transmissions from the
eNB (e.g., at least one transmission every X=10 or 20 ms) for AGC
maintenance and fine time/frequency tracking.
For unlicensed carrier operation, the eNB may use any one of the
following approaches to ensure that activated UEs have enough
reference signals for AGC maintenance and fine time/frequency
tracking. For example, in one embodiment for unlicensed carrier
operation, the eNB may transmit discovery signals more frequently
than what is considered standard. More particularly, the eNB may
transmit discovery signals once every 10 ms to 20 ms. However this
embodiment's approach may most likely lead to increased
interference on the unlicensed carrier frequency and increased eNB
power consumption since the transmitted discovery signals are
typically always transmitted by the eNB to facilitate cell
detection.
In another embodiment, when the eNB determines that it has not made
any transmissions on an unlicensed carrier for some time (e.g., for
about 10 to 20 ms), the eNB will not schedule PDSCH to an activated
UE on that carrier, until the eNB transmits a few subframes (e.g.,
1 or 2 subframes) containing at least reference signals (e.g.,
CRS), and then resume PDSCH transmissions to the UE thereafter.
Here, the eNB may optionally send an indicator on the PCell (e.g.,
special PDCCH/EPDCCH message) that tells the UE whether reference
signal transmissions (e.g., CRS) are present on the Scell for a
particular subframe or a set of subframes.
In other embodiments for unlicensed carrier operation, the eNB and
the activated UE can maintain a DL time synchronization timer. The
DL time synchronization timer can be set to a value Xms (e.g., X=10
or 20 ms, the value of X depends on time for which UE can
autonomously maintain time-frequency sync without receiving any eNB
transmissions). The subframe in which the DL time synchronization
timer is set to Xms can be any subframe in which the eNB transmits
PDSCH to the UE (from UE perspective, it will be any subframe where
PDSCH is received from the eNB). Alternately, the subframe can be
any subframe where the UE transmits a positive acknowledgement
(HARQ-ACK) in response to a PDSCH received from the eNB (in this
case, from eNB perspective, it will be any subframe in which a
positive acknowledgement is received from the UE). The DL time
synchronization timer is then counted down in subsequent subframes.
When the DL time synchronization timer expires (i.e., when it
becomes 0), the eNB determines that the UE may not have fine time
frequency synchronization. Although, the UE may not have fine time
frequency synchronization, the UE can continue to monitor Scell
subframes for control signaling, as it may be able to decode some
PDCCH/EPDCCH and/or PDSCH messages (albeit with a higher bit error
rate) When the eNB determines the UE may not have fine time
frequency synchronization, the eNB can schedule PDSCH transmissions
with a conservative PDSCH assignment (e.g., using MCS corresponding
to QPSK and one spatial layer instead of 64 QAM and multiple
spatial layers), for a few subframes (e.g., 1 or 2 subframes) until
the UE regains fine time/frequency sync to decode more aggressive
PDSCH assignments. This alternative avoids the overhead of
re-activation command transmission on the PCell (described below).
In the alternative, the eNB can send an activation command (in this
example, a re-activation command) to the UE on the PCell. Upon
reception of the activation command on the Pcell in subframe n, the
UE determines that reference signal transmissions are present in at
least subframes (n) to n+p (e.g., p=1 or 2) and regains fine
time-frequency sync using the signals in those subframes.
In yet another embodiment for unlicensed carrier frequency
operation, the eNB may also reuse a deactivation timer as a `DL
time synchronization timer` described above (i.e., the eNB
configures the UE with a deactivation timer of Xms, and resends the
activation command whenever the timer expires (if there is still
data in the buffer for transmission to that UE)). In this
embodiment situation, the UE deactivates the Scell when the timer
(deactivation timer/DL time synchronization timer) expires.
Although the preferred embodiments have been described herein in
detail, it should be understood that various changes, substitutions
and alterations can be made therein without departing from the
teachings, understanding and scope of the invention as defined by
the appended claims.
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